Method of vulcanizing rubber



Patented Apr. 2, 1963 ice 3,084,115 NIETHOD F VULCANIZING RUBBER Wendell V. Smith, Nntley, N.J., and Verne G. Simpson,

This invention relates to the vulcanization of rubber by exposing it to ionizing radiation, and more particularly it relates to the acceleration or enhancement of such vulcanization by having certain acrylate, diacrylate or dimethacrylate compounds present in the rubber during such exposure to ionizing radiation.

It is known to the art that rubber may be at least partially if not completely vulcanized when placed in a field of ionizing radiation. There are, however, limitations on this process. First, after a certain amount of exposure to such radiations, rubber has been observed to degrade.

Second, the cost of subjecting a material to irradiation varies directly with the quantity of energy that must be used and this cost is quite high, such that presently, radiation vulcanization cannot compare in cost to ordinary heat vulcanization.

This invention shows that when small quantities of certain acryla-te, diacrylate or dimethacrylate compounds are mixed in with natural rubber or SBR (i.e., styrenebutadiene rubber, formerly known as GR-S or Buna-S) the amount of ionizing radiation necessary to effect cure is substantially reduced as shown in the working examples and tabulated results below.

The term ionizing radiation is used here in its conventional sense as referring to radiation which when absorbed by matter produces ionization. Well known to the art as ionizing radiations are X-rays, gamma rays, high speed electrons, beta rays, and high speed protons, alpha particles and fast neutrons. Of these, X-rays, gamof at least about 8 electron volts. For more details of such energy, reference may be had to Table 10a on pages 105-115 of Electron 'Impact Phenomena and Properties of Gaseous Ions, by F. H. Field and I. L. Franklin (Academic Press, Inc., 1957). Energy sources for such high energy ionizing radiation may be electron accelerators of the Van de Graatf or linear types or cobalt sources or nuclear reactors, etc. The permissible irradiation dosage is from 4 to 40 watt hours per pound of material with a dosage of from 4 to 20 Watt hours per pound preferred. This permissible dosage is equivalent to 3.2)(10 to 32x10 rad.

For some as yet unexplained reason, methacrylates are ineffective in achieving the desired results while both acrylates and dimethacrylates as well as diacrylates are effective.

The most significant data in terms of pointing out the novelty of this invention resides in observations made of the stress at elongations of 200% (200% modulus). These data are summarized as follows in Table 1. All samples contain about by weight of carbon black (commercial material known as Philblack O, supplied by Phillips Petroleum Co.). Further properties and exact compositions are listed'in Tables 2-10. The carbon black is optional. The invention is applicable to any given SBR or Hevea composition, whether filled or unfilled. If desired any conventional filler may be employed to give its usual effects.

The desired compositions were mixed on a cold mill and test slabs of 6 /2 x 6 /2 x 0.1" were molded in a press for 5 minutes at 260 F. The slabs were then irradiated either in an X-ray machine or by exposure to a beam of 2 million volt electrons obtained from a Van de Graaif accelerator. To prevent undue heating of samples, intermittent exposures were used; the total dosage being controlled by the total time of exposure. Tests on the treated slabs were carried out by using conventional rubber testing equipment.

TABLE 1 Stress (p.s.i.) at 200% Elongation for Composition Containing Various Additives Base rubber SBR and Philblack O Smoked sheet and Phllblack 0 Radiation dose, watt hours per pound Additive None (control) 50 100 120 200 400 2 pts. ethylene diacry1ate 50 130 5 pts ethylene d1acrylate 50 240 10 pts. ethylene diacrylate 50 350 2 pts ethylene dimethacrylate 170 5 pts ethylene dimeth acrylate-. 50 240 10 pts. ethylene dimethacrylate. 50 400 1 225 1 660 1 1450 20 pts. ethylene dimethaerylate. 50 400 10 pts. octyl methacrylate 50 80 130 370 20 pts. oetyl methacrylate 50 70 180 250 10 pts. tetramethylene diacrylate- 40 570 450 930 1, 830 2 pts. glyceryl ttiacrylate 50 100 170 250 500 5 pts. glyceryl triacrylate 1 100 170 250 450 10 pts. lauryl acrylate 35 200 450 700 20 pts. lauryl acrylate L. 20 80 10 pts. polyethylene glycol dimethacrylate, 2 mol.

weight about 340 30 1 Dosages at 0, 4, 8, and 16 Weight hours/pound.

2 300% modulus with dosages of 0, 4, 10, 20. Control values of modulus are 46, 138, 225, 645 psi. 3 Commercial material sold as Monomer MG-l by Union Carbide and Carbon 00.

ma rays and high speed electrons (e.g., accelerated through 200,000 volts or more) are particularly suitable to this invention. In general, the operative radiation may be described as high energy radiation, the individual particles or photons of which possess energies From these data it may readily be observed that significant results are obtained when .the following compounds are employed.

(a) Alkyl :acrylates.

(b) Acrylic esters of saturated dihydric alcohols.

() Methacrylic esters of saturated dihydric alcohols.

(d) Acrylic and methacrylic esters of polydihydric alcohols.

It was observed that glyceryl triacrylate is not effective so that all vpolyhydric alcohol esters of acrylic acid may not be generally claimed.

The amount of additive used is in all cases at least 1 /2 parts, per 1100 parts of rubber. The amount used typically varies with the amount of active component in the compound. There should be at least 1 /2 parts of acrylate or methacrylate component per 100 parts of rubber. In the case of ethylene diaerylate, where the acrylate component is about 74% of the molecule, 2 parts of the compound are sufiicient, while in the case of octyl acrylate, the compound contains only about 40% acrylate hence at least 4 parts are used. Above about 15 parts of acrylate or methacrylate component per 100 parts of rubber, there is very little increase obtained by adding greater quantities of accelerator. As aforesaid, alkyl monomethacrylates are ineffective while dimethacrylates are effective.

The preferred additives are:

Alkyl acrylates/alkyl from C to C Diacrylates and dimethacrylates of polyethylene glycol, having a molecular weight of from about 170 to 1000. Diacrylates and dimethacrylates of ethylene, propylene and tetramethylene glycols.

The following Tables 2 to 10 illustrate the invention in more detail.

TABLE 2 SBR With Ethylene Diacrylate and Control Stock No. 3 4 SBB 1500. 100 100 100 Philblaek 50 50 55 Ethylene die 2 5 Tensile strength, lbs/ill.

Radiation dose, wt. hrs./lb.:

Elongation, percent 200% modulus, lbs. /in 1 Durometer Torsional hysteresis at room temperature Torsional hysteresis at 280 F.

TABLE 3 SBR With Ethylene Dimethaclylate Stock No 5 6 7 8 SBB 1500.... 100 100 100 100 Philblack O 50 Ethylene dimethacrylate 2 5 10 20 Tensile strength, lbs/in.

Radiation dose, weight hrs.[lb.:

Elogation, percent 200% modulus, lbs./iu.'-

Durometer Torsional hysteresis at room temperature TABLE 4 SBR With Octyl Acrylate and Octyl M ethacrylate Stock No 9 10 11 12 SEE 1500- 100 100 100 Philblaek O--- 55 50 55 00 Oetyl aerylate 10 20 Octyl methacrylete 10 20 Tensile strength, lbs/in.

Radiation dose, weight hrs./1b.:

Elongation, percent 200% modulus, lbs/in.

Durorneter 5 TABLE 4-Continued Smoked Sheet With Octyl Acrylwte and Daryl Meth- Stock N 9 10 11 12 SBB 1500 100 100 100 100 acrylate and Control Philblack O 55 50 55 60 Octyl acrylate 10 20 Octyl methaerylate Stock N o 22 23 24 25 26 Rubber SS 100 100 100 100 100 Philblack O 50 55 60 55 60 Torsional hysteresis at room Octyl acrylate 10 20 temperature Ogtyl methacrylnte 10 20 Tensile strength, lbs/in. Rizgiation dose, weight hrsJ Elongation, percent 200% modulus, lbs/in. BLE TA 5 25 40 0 6g 40 50 SBR With Acrylate and Methacrylate and Control I 38 2 8 2%, 58 $8 17.2 400 600 930 370 250 CURED WITH 12 \VATT HR. LB. ELECTRON DOSAGE Durometer 1 0 1) 1 0 0 1 0 0 1 0% S 15 Philback 0 50 60 e0 00 Octyl methacryla 2 36 45 50 36 37 Octyl methaorylate (sample 2) 20 45 51 59 43 42 Octyl acrylate. 20

Show A dmometer hardness h 52 50 48 60 Torswnal hysteres1s at room temperature Tensile Strength, 1b./in 2 1, 850 1, 760 1, 550 2 310 Elongation, percent 590 610 540 270 200% modulus, Ill/i1 50 300 300 400 4 517 Torsional hysteresis at 280 F.

TABLE 6 303 370 SBR With Calcium and Lamyl Acrylates and Control Stock No 17 20 21 SBR 1500 1500 TABLE 8 Philblack O 5 Calcium acrylate. R0 Smoked Sheet and SBR Wzth Monomer M G] and Lauryl acrylate 10 20 Tetramethylene Diacrylate Stock N0 27 28 29 '30 SEE 1500 100 100 46 8 41 35 20 Rubber ss 100 100 1 8 47 5 5 8 55 Philblaok o 55 55 55 55 225 440 380 500 275 M G-1 1 10 10 5 6 040 030 1,150 Tetramethylene diaerylate 10 10 Space A durometer hardness Tensile strength, m z

Tensile strength, p.s.l.

Elongation at break, percent 200% modulus, lbs/in.

Torsional hysteresis at room temperature Torsional hysteresis at 280 F.

4 a .1sa .287 as .234 .104 .290 .218 17.2 .157 .067 .215 .117

1 Dimethacrylate of polyethylene glycol (Carbide and Carbon).

TABLE 9 Smoked Sheet and SBR With Glyceryl Triacrylate Stock No 31 32 33 34 Rubber SS 100 100 SBR 150[)- 100 100 Philblaek 50 50 50 50 Glyeeryl triacrylate 2 2 5 Radiation dose, weight hrs/1b.:

Elongation, percent 200% modulus, lbs/in.

Durorneter Torsional hysteresis at room temperature Torsional hysteresis at 280 F.

TABLE 10 Pale Crepe With Octyl Acrylate and Control Stock No 35 36 Pale crep" 100 Ihilblaek O 50 50 Octyl acrylato 20 200% modulus Tensile strength Elongation, percent Shore A dnromctor hardness In addition to the enhancement of the 200% modulus properties obtained through this invention, tensile strength and torsional hysteresis properties are not adversely aifected. For example, irradiation of a sample (Table 3, Stock No. 7) containing 10 parts of ethylene dimethacrylate per 100 parts SBR and 55 parts carbon black at 8.6 watt hours per pound (7X10 rad.) yielded a product that developed a stress of 750 psi. at 200% elongation. This should be compared with a comparably exposed sample (Table 2, Stock No. 1), identical in composition except for it not containing any additive. This latter sample developed a stress at 200% elongation of only 200 p.s.i. while non-irradiated samples both with and without additives developed only 50 p.s.i. when stretched to 200% elongation. The enhancement by 275% in its modulus achieved by Stock No. 7 as compared to Stock No. 1 was achieved not only without any reduction in tensile strength but with a 76% increase in tensile strength.

It is to be noted that use of only 2 parts of ethylene dimethacrylate at 17.2 watt hours per pound of ionizing radiation yields a sample (Table 3, Stock No. 5) having a stress at 200% elongation of 700 pounds per square inch, 9. increase over the value obtained with the comparably exposed additive-free sample While there is an increase of about 5% in the tensile strength observed.

It has been indicated above that the cost of subjecting a material to cure by means of ionizing radiations increases with the amount of radiation necessary. The economical aspect of this invention may be observed by noting that in order to cure an SBR sample sufficiently so as to obtain a 200% modulus of 400 pounds per square inch, an exposure to electrons of 17.2 watt hours per pound is required. Where only 5 parts by weight of ethylene dimethacrylate are premixed with 100 parts of SBR, an exposure of less than 8.6 (probably about 7) watt hours per pound is sufiicient to achieve this same result. Thus the cost of irradiation is more than halved through the use of only a minor amount of a relatively inexpensive chemical.

Comparable examples Will be noticed in the tables wherein various compounds are shown to have been tested in conjunction with SBR (GR-S), smoked sheet and pale crepe. It has been ascertained that no enhancement is achieved when these activating compounds of this invention are used in conjunction with neoprene, Paracril, or Butyl rubbers. Some slight enhancement of properties has been obtained with polyethylene and polyvinyl chloride but this enhancement is at best marginal and of little significance.

One sample of 100 parts of smoked sheet containing 55 parts of Philblack O and parts of tetramethylene diacrylate was exposed to X-rays equivalent to 6 watt hours per pound and was observed to have a 200% modulus of 430 pounds per square inch as opposed to a comparably exposed sample containing only 100 parts smoked sheet and 50 parts Philblack O and which gave a 200% modulus value of 230 pounds per square inch. Any other conventional source of ionizing radiation may be substituted for electrons or X-rays, with equivalent results.

Having thus described our invention, what we claim and desire to protect by Letters Patent is:

1. A method of vulcanizing a rubber composition, com.- prising subjected said composition to a dosage of from 4 to 40 Watt hours of high energy ionizing radiation per pound of said composition, the said composition comprising 100 parts by weight of a rubber selected from the group consisting of Hevea rubber and SBR, and, as an accelerator, from 1.5 to parts of a material selected from the group consisting of alkyl acrylates, acrylic esters of saturated dihydric alcohols, methacrylic esters of saturated dihydric alcohols, acrylic esters of polydihydric alcohols, and 'methacrylic esters of polydihydric alcohols.

2. A method of vulcanizing a rubber composition, comprising subjecting said composition to a dosage of from 4 to 'Watt hours of high energy ionizing radiation per pound of said composition, the said composition comprising 100 parts by weight of a rubber selected from the group consisting of Hevea rubber and SBR, and, as an accelerator, *from 1.5 to 15 parts of a material selected from the group consisting of alkyl acrylates in which the alkyl group has from 4 to 12 carbon atoms, polyethylene glycol diacrylate, polyethylene glycol dimethacrylate, ethylene diacrylate, ethylene dimethacrylate, propylene diacrylate, propylene dimethacrylate, tetramethylene diacrylate and tetramethylene dimethacrylate.

3. A method as in claim 2, in which the said accelerator is tetramethylene diacrylate.

4. A method as in claim 2, in which the said accelerator is octyl acrylate.

5. A method as in claim 2, in which the said accelerator is ethylene diacrylate.

6. A vulcanized rubber composition comprising parts by weight of a rubber selected from the group consisting of Hevea rubber and SBR, and, as an accelerator, from 1.5 to 15 parts of a material selected from the group consisting of alkyl acrylates, acrylic esters of saturated dihydric alcohols, methacrylic esters of saturated dihydric alcohols, acrylic esters of polydihydric alcohols and methacrylic esters of polydihydric alcohols, said composition having been vulcanized by the method of claim 1.

7. A vulcanized rubber composition, comprising 100 parts by weight of a rubber selected from the group consisting of Hevea rubber and SBR, and, as an accelerator, from 1.5 to 15 parts of a material selected from the group consisting of alkyl acrylates in which the alkyl group has from 4to 12 carbon atoms, polyethylene glycol diacrylate, polyethylene glycol dimethacrylate, ethylene diacrylate, ethylene dimethacrylate, propylene diacrylate, propylene dimethacrylate, tetramethy-lene diacrylate, and tetramethylene dimethacrylate, said composition having been vulcanized by the method of claim 2.

8. A composition as in claim 7, in which the said accelerator is tetramethylene diacrylate.

9. A composition as in claim 7, in which the said accelerator is octyl acrylate.

10. A composition as in claim 7, in which the said accelerator is ethylene diacrylate.

References Cited in the file of this patent UNITED STATES PATENTS 2,155,590 Garvey Apr. 25, 1939 2,505,067 Sachs et al Apr. 25, 1950 2,609,353 Rubens Sept. 2, 1952 2,668,133 Brophy et a1. Feb. 2, 1954 2,670,483 Brophy Mar. 2, 1954 2,973,309 Brodkey et a1 Feb. 28, 196 1 FOREIGN PATENTS 546,816 Belgium Oct. 6, 1956 732,047 Great Britain June 15, 1955 OTHER REFERENCES Lawton et al.: Nature, vol. 172, pp. 76, 77, July 11, 1953.

Maran's: Abstracts of Papers of Am. Chem. Soc., 132nd meeting, page 21T, September 8-13, 1957.

Mesrobian: Abstracts of Papers of Am. Chem. Soc., 132nd meeting, page 19Q, September 8-13, 1957. 

1. A METHOD OF VULCANIZING A RUBBER COMPOSITION, COMPRISING SUBJECTED SAID COMPOSITION TO A DOSAGE OF FROM 4 TO 40 WATT HOURS OF HIGH ENERGY IONIZING RADIATION PER POUND OF SAID COMPOSITION, THE SAID COMPOSITION COMPRISING 100 PARTS BY WEIGHT OF A RUBBER SELECTED FROM THE GROUP CONSISTING OF HEVEA RUBBER AND SBR, AND, AS AN ACCELERATOR, FROM 1.5 TO 15 PARTS OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OF ALKYL ACRYLATES, ACRYLIC ESTERS OF SATURATED DIHYDRIC ALCOHOLS, METHACRYLIC ESTERS OF SATURATED DIHYDRIC ALCOHOLS, ACRYLIC ESTERS OF POLYDIHYDRIC ALCOHOLS, AND METHACRYLIC ESTERS OF POLYDIHYDRICC ALCOHOLS. 